1. Field of the Invention
Generally, the invention relates to the etching of the gate material in the fabrication of field-effect transistors.
2. Description of the Related Art
In the fabrication of MOS transistors, polysilicon or an alloy of silicon and germanium is generally used as the gate material. This material is deposited on a thin layer of thermal oxide, also called gate oxide, and then etched to the dimensions of the gate.
The etching, by any means whatsoever, may be carried out using a hard mask which defines the region intended subsequently to form the gate and which protects the said region during the operation. An inorganic material, such as silicon dioxide or silicon nitride, is therefore normally used for the hard mask. The absence of carbon in the material of which the mask is composed, responsible for destroying the gate oxide layer when etching the gate, makes it possible to significantly increase the selectivity of the etching of the gate material made of polycrystalline silicon or of a silicon-germanium alloy with respect to the gate oxide.
Nevertheless, the use of these conventional masks has drawbacks which impair the quality of the transistors fabricated.
It is known to isolate the future active regions of a semiconductor device in a conventional manner (LOCOS) or by shallow trenches (STI or Shallow Trench Isolation) using silicon dioxide. When the hard mask for etching the gate is made of silicon dioxide, its removal runs the risk of cutting away the trenches or of destroying the isolation region because it is of the same nature as the isolation oxide.
Moreover, using a hard mask based on silicon nitride runs the risk of destroying the gate material when removing the mask. This is because the process of etching silicon nitride is very similar to that for silicon, which results in poor etching selectivity between these 2 materials and in detection of the end of etching of the hard mask that occurs at the gate not being sufficient to prevent partial destruction of the gate lying beneath the hard mask.
It would therefore seem to be necessary to develop a process for etching the gate material which does not have the abovementioned drawbacks.